When transmitting high data rate digital signals, certain phenomena related to the propagation of these signals may disturb the transmission and thus give rise to errors in the transmitted message. A signature measurement serves to qualify how well a radio link, for example, will withstand this type of propagation defect which is attributed to "multiple paths" that appear when a transmitted radio signal follows a plurality of propagation paths. In general, in order to use this signature measurement, use is made of a "two path" model.
In an article published in "International Conference on Measurements for Telecommunication Transmission Systems" MTTS 85 (conference publication No. 286), entitled "Automated Signature Measurement of Operational Microwave Radio-relay Equipment Using a Noval Multipath Simulator", G. D. Richman of British Telecommunications Research Laboratories, U.K., present knowledge relating to multiple paths, to signature measurements, and to multipath simulators is described. The purpose of such simulators is to produce static and dynamic test conditions for fully characterizing the capacity of digital equipment to attenuate distortion due to abnormal propagation effects.
Thus, in the prior art, use is made of a multipath simulator which may operate either in baseband, or at intermediate frequency, or at microwave frequency. These various types of construction suffer from various drawbacks, such as:
in baseband, the analog multipliers used in such a simulator need to be reliable over a wide bandwidth and they need to be insensitive to temperature;
at intermediate frequency, the main problem relates to the phase shifter used in the simulator which must be a wideband phase shifter; and
at microwave frequency, the equipment is often bulky and expensive.
In all cases, a multipath simulator is a complex and expensive apparatus that needs to be installed on a link each time it is desired to measure the signature of a piece of equipment.
The object of the invention is to mitigate these drawbacks.